1. Field of the Invention
The present invention relates to an organic electroluminescent display device (hereinafter referred to as an xe2x80x9corganic EL display devicexe2x80x9d) fabricated by using an organic electroluminescent layer (hereinafter referred to an xe2x80x9corganic EL layerxe2x80x9d) and to a method for manufacturing the same.
2. Description of the Related Art
An organic electroluminescent device is comprised of an anode, a cathode and an ultra thin-film organic EL layer composed of an organic electroluminescent compound disposed between the anode and the cathode. When a voltage is applied between the anode and the cathode, a hole and an electron from the anode and the cathode are injected respectively into the organic EL layer and are recombined therein. At this point, energy is produced which causes a molecule of an organic electroluminescent compound constituting the organic EL layer to be pumped and electroluminescence occurs while the pumped molecule is deactivated to a ground state. The organic EL display device is a layer-stacked device utilizing this electroluminescent phenomenon.
The organic EL layer has a single layer structure or a multi-layered structure containing at least one of organic layers including an organic layer called an organic electroluminescent layer which emits light when holes and electrons are combined therein, an organic layer called a hole transporting layer into which holes are easily injected and in which electrons are hardly moved, and an organic layer in which electrons are easily injected and in which holes are hardly moved.
The thin-film constituting the organic EL device is ordinarily formed by a vacuum evaporation method and a thickness of the film is not more than 1 xcexcm. There is, however, a problem in that, if water, acids or alkali solutions are used while these thin films are formed, since they soak into an interface between organic EL layers, and also between the organic EL layer and the cathode, electroluminescent performance and life characteristics of the organic EL layer are remarkably degraded.
Therefore, photolithography using a conventionally known wet-etching technique can not be used for high definition separating process of the organic EL layer.
Moreover, a dry-etching technique also cannot be used because a solvent, developer or release agent used for this technique may cause damage to the organic EL layer or the cathode during processes of applying a resist, of development and of releasing.
To manufacture a color display device using such organic EL devices, roughly three kinds of methods are available conventionally.
A first method is to use an organic EL device emitting white light which passes through a color filter having primary colors to achieve color displaying. The color displaying technology using such white layer-stacked devices and color filters is disclosed in, for example, Japanese Laid-open Patent Application No. Hei8-96959.
This first method disclosed above is the same in terms of technological principles as that used widely for transmission type color liquid crystal displays. Moreover, according to this method disclosed, a color filter with high definition can be easily implemented by using photolithography technology and a patterning of the organic layer is not required if only one kind of the organic EL device to emit white light is prepared. Therefore, there is an advantage in that color display devices can be easily manufactured by this method.
However, the above first method has a disadvantage in that, since light having an unwanted wavelength is removed from emitted white light with a filter, a major portion of the light emitted from the organic EL device is discarded wastefully, thus making it difficult to effectively utilize the electroluminescence of the organic EL device.
Furthermore, to put this first method to practical use, it is necessary to develop white electroluminescent organic materials having a high electroluminescent efficiency rate and a long life, however, the actual development of such organic materials is extremely difficult.
A second method is to use a color conversion technique employing a blue electroluminescent organic EL device and color conversion layers. A color display technique of the organic EL device using this color conversion method is disclosed in, for example, Japanese Laid-open Patent Application No. Hei3-152897.
In this color conversion method disclosed, blue light with a short wavelength is produced by using the organic EL device and then green light and red light each having a wavelength longer than that of the blue light is produced by causing the blue light to pass through the color conversion layer. If this method is used, it is not necessary to scale down the organic EL device and the color conversion layer with high definition can be implemented by using photolithography, thus enabling easy fabrication of the color display device with high definition.
However, this second method also has a disadvantage that it is very difficult to actually increase the conversion rate of the color conversion layer and therefore the efficiency rate of effectively using electroluminescence of the organic EL device is low.
Moreover, in this second method, it is also difficult to obtain flatness of a surface of the color conversion layer when its patterning is carried out and it is difficult to carry out the patterning of a flattened layer without breaking the transparent electrodes.
To overcome such shortcomings as described above in implementing color display devices, a method for effectively using electroluminescence of the organic EL device for displaying is proposed in which organic EL devices each having one of primary colors of red, green and blue are formed independently and arranged.
In view of the above, it is an object of the present invention to provide an organic electroluminescent display device and a method for manufacturing the same wherein a patterning of the organic electroluminescent layer can be carried out without a process of photolithography or metal masking and wherein the patterning of large area and/or with high definition is made possible.
According to a first aspect of the present invention, there is provided a method for manufacturing an organic electroluminescent display device comprising:
a first process of forming a patterned transparent electrode on an anode board;
a second process of forming a hole transporting layer on said transparent electrode;
a third process of forming three kinds of layer-stacked devices by sequentially stacking a metal cathode, organic electroluminescent layers each emitting red, green or blue color light on a surface of a piece of strip-like metal;
a fourth process of arranging the above three kinds of layer-stacked devices with each of the organic electroluminescent layers facing upward on an insulation board;
a fifth process of overlaying the anode board and the insulation board each other so that the transparent electrode and the layer-stacked devices intersect each other and that the hole transporting layer and the organic electroluminescent layer are opposed to each other; and
a sixth process of making sealing around the anode board and the insulation board being overlaid each other.
In the organic EL display device of this aspect of the present invention, the patterning of the organic electroluminescent layer constituting the layer-stacked device can be carried out without a process of photolithography or metal masking, thus allowing an easy production of color organic EL display devices. Moreover, since the patterning of the organic electroluminescent layer can be carried out without the use of a shadow mask such as a metal mask, the patterning of large area and/or with high definition is made possible.
In the foregoing, a preferable mode is one wherein the third process further comprises a step of stacking a metal cathode and an organic electroluminescent layer emitting red, green or blue light on a surface of a piece of strip-like metal to form the layer-stacked device. By forming the layer-stacked device, less materials become waste compared with cases of using the photolithography or metal masking, thus reducing production costs. Moreover, since the organic electroluminescent layer is separately evaporated for every electroluminescent color, a material for each electroluminescent color is not mixed during evaporation, thus avoiding mixing of colors. Also, each of defective layer-stacked devices can be individually replaced even in the course of manufacturing, thereby reducing losses caused by defective layer-stacked devices, increasing yield of products and resulting in the reduction of production costs. Furthermore, because a metal strip can be used as a cathode, wiring resistance for the cathode can be extremely lowered.
Also, a preferable mode is one that wherein the metal electrode has a low work function.
Furthermore, a preferable mode is one that wherein further comprises a step of forming bumps and dips on the surface of a piece of the metal to cause light to be reflected irregularly.
According to a second aspect of the present invention, there is provided an organic electroluminescent display device comprising:
a plurality of line-like transparent electrodes spreading in a first direction;
an anode board having a hole transporting layer formed on said transparent electrode;
an insulation board disposed opposite to said anode board; and
three kinds of layer-stacked devices having organic electroluminescent layers each emitting a different color light, which are disposed between the anode board and the insulation board;
whereby each of the three kinds of layer-stacked devices is composed of a piece of strip-like metal, a metal cathode formed on a surface of a piece of the metal and organic electroluminescent layers, formed on the metal cathode, each emitting a red, green or blue color light, wherein the organic electroluminescent layers are arranged on the side of said anode board in a second direction in which a longitudinal direction of the organic electroluminescent layers intersects the first direction.
In the foregoing, it is preferable that the layer-stacked device is composed of a piece of strip-like metal, a metal cathode formed on a piece of the metal and an organic electroluminescent layer formed on the metal cathode emitting red, green or blue light.
Also, a preferable mode is one that wherein the metal electrode has a low work function.
Furthermore, it is preferable that bumps and dips are formed on a surface of a piece of the metal to cause light to be reflected irregularly.